1. Field of the Invention
The present invention relates to a voltage controlled oscillator (VCO), and more particularly, to a voltage controlled oscillator (VCO) connected and disposed in a phase locked loop (PLL) and capable of switching oscillation frequency bands by supplying a band switching signal to an LC resonator circuit.
2. Description of the Related Art
Hitherto, there are known, as the mobile telecommunication systems used in respective countries in the world, DCS (Digital Cellular system ) used in England, Germany, Italy, France, and some countries of Asia and GSM (Global System for Mobile Communications) used in Europe, the United States, Africa, and some countries in Asia as European Standard System for digital hand-held phones. Since DCS and GSM are mobile telecommunication systems employing a different telecommunication system, users must be provided with two hand-held phones, that is, a hand-held phone used for DCS (hand-held phone dedicated for DCS) and a hand-held phone used for GSM (hand-held phone dedicated for GSM) in order to subscribe to both of DCS and GSM.
However, since both of DCS and GSM use GMSK (Gaussin Minimum Shift keying) as a signal modulation system while they employ a different communication system, they are different from each other only in that their usable frequency band is different. Accordingly, hand-held phones which can be commonly used by both of DCS and GSM are being developed in place of using two hand-held phones dedicated for DCS and GSM and several commonly usable hand-held phones have been proposed.
These previously proposed hand-held phones which are commonly usable in DCS and GSM include two voltage controlled oscillators, that is, a first voltage controlled oscillator for generating a local oscillation signal suitable for a DCS band receiving frequency and a second voltage controlled oscillator for generating a local oscillation signal suitable for a GSM band receiving frequency, and they selectively use the first and second voltage controlled oscillators based on whether they are used in DCS or in GSM. These previous commonly usable hand-held phones use the two voltage controlled oscillators as well as connect them to individual phase locked loops, which inevitably increases the size of the commonly usable hand-held phones as well as increases the number of parts thereof, which makes the cost of the previous commonly usable hand-held phones expensive.
Hand-held phones commonly usable in the DCS and the GSM, which were proposed after the development of the previous hand-held phones to improve the disadvantages of them (improved-type hand-held phones), employ one voltage controlled oscillator connected to one phase locked loop and are switched to DCS and GSM by switching the resonance circuit of the voltage controlled oscillator in response to a band switching signal. That is, the switching of the resonance circuit in response to the band switching signal permits the voltage controlled oscillator to output a local oscillation signal having an oscillation frequency suitable for the receiving frequency of the DCS band and a local oscillation signal having an oscillation frequency suitable for the receiving frequency-of the GSM band, respectively.
FIG. 4 is a block diagram showing an example of the arrangement of a phase locked loop to which one voltage controlled oscillator used in an improved-type hand-held phone is connected, and FIG. 5 is a circuit diagram showing an example of the arrangement of the main portion of the voltage controlled oscillator connected to the phase locked loop shown in FIG. 4.
As shown in FIG. 4, the phase locked loop (PLL) includes a voltage controlled oscillator (VCO) 31, a loop filter (LPF) 32, a phase comparison circuit (COMP) 33, a reference frequency signal input terminal 34, and an oscillation signal output terminal 35.
The output terminal (oscillation signal output terminal) of the voltage controlled oscillator 31 is connected to the oscillation signal output terminal 35 and to one input terminal of the phase comparison circuit 33, and the input terminal (control voltage input terminal) thereof is connected to the output terminal of the loop filter 32. The output terminal of the phase comparison circuit 33 is connected to the input terminal of the loop filter 32, and the other input terminal thereof is connected to the reference frequency signal input terminal 34.
As is known well, the operation of the phase locked loop is such that the phase comparison circuit 33 compares the phase of the oscillation signal of the voltage controlled oscillator with the phase of a reference frequency signal, outputs an error signal indicating the error between the phases of the oscillation signal and the reference frequency signal, and supplies the error signal to the loop filter 32. The loop filter 32 smooths the inputted error signal, outputs it as a control voltage and supplies the control voltage to the LC resonator circuit of the voltage controlled oscillator. In the voltage controlled oscillator 31, the resonance frequency of the LC resonator circuit is controlled in correspondence to the control voltage supplied thereto, whereby the frequency of an oscillation signal is adjusted to a normal frequency.
As shown in FIG. 5, the voltage controlled oscillator (VCO) 31 includes the LC resonator circuit 41, an amplifier stage 42, a control voltage input terminal 43, an oscillation signal output terminal 44, a band switching signal input terminal 45, a shunt capacitor 46, a series inductor 47, a coupling capacitor 48, and a series resistor 49. In this case, the LC resonator circuit 41 includes a varactor diode 411, a first inductor 412, a second inductor 413, a first direct current preventing capacitor 414, a second direct current preventing capacitor 415, and a switching diode 416. Note that, the same reference numerals as used in FIG. 4 are used in FIG. 5 to denote the same components.
In the LC resonator circuit 41, the anode of the varactor diode 411 is connected to the ground, the cathode thereof is connected to one end of the first direct current preventing capacitor 414 and to the other end of the series inductor 47. One end of the first inductor 412 is connected to the other end of the first direct current preventing capacitor 414 and to one end of the coupling capacitor 48, and the other end thereof is connected to one end of the second inductor 413 and to one end of the second direct current preventing capacitor 415. The other end of the second inductor 413 is connected to the ground. The other end of the second direct current preventing capacitor 415 is connected to the anode of the switching diode 416 and to one end of the series resistor 49. The cathode of the switching diode 416 is connected to the ground. Further, the input terminal of the amplifier stage 42 is connected to the other end of the coupling capacitor 48, and the output terminal thereof is connected to the oscillation signal output terminal 44. In addition to the above, one end of the shunt capacitor 46 is connected to the control voltage input terminal 43 and to one end of the series inductor 47. The other end of the shunt capacitor 46 is connected to the ground. The other end of the series resistor 49 is connected to the band switch signal input terminal 45.
The voltage controlled oscillator 31 arranged as described above will operate as follows. When the voltage controlled oscillator 31 is used as the local oscillator for DCS, a band switching signal of a positive voltage is supplied to the band switching signal input terminal 45. The band switching signal of the positive voltage is applied to the anode of the switching diode 416 through the series resistor 49 to thereby turn on the switching diode 416 so as to electrically short circuit the second inductor 413. Further, the control voltage supplied to the control voltage input terminal 43 is supplied to the varactor diode 411 through the series inductor 47 so that the capacitance value of the varactor diode 411 is varied in correspondence to the magnitude of the control voltage. At this time, the LC resonator circuit 41 is parallel-resonated with the inductance value of the first inductor 412 and the series capacitance value of the varactor diode 411, which is connected in parallel to the first inductor 412, and the first direct current preventing capacitor 414, and an oscillation signal (in a high frequency band, for example, from 1450 MHz to 1500 MHz), which corresponds to the parallel resonance frequency, is generated to the amplifier stage 42 and supplied to the oscillation signal output terminal 44 from the output terminal thereof.
Next, when the voltage controlled oscillator 31 is used as the local oscillator for GSM, a band switching signal of a ground voltage is supplied to the band switching signal input terminal 45. Since the band switching signal of the ground voltage only makes the anode voltage of the switching diode 416 as large as the cathode voltage thereof, the switching diode 416 is turned off so that the second inductor 413 is connected in series to the first inductor 412. Further, the control voltage supplied to the control voltage input terminal 43 is supplied to the varactor diode 411 through the series inductor 47 similarly to the above mentioned case and varies the capacitance value of the varactor diode 411 in correspondence to the magnitude of the control voltage. At this time, the LC resonator circuit 41 is parallel-resonated with the overall inductance value of the first inductor 412 and the second inductor 413, which are connected in series to each other, and the series capacitance value of the varactor diode 411, which is connected to the series circuit of the first and second inductors 412 and 413, and the first direct current preventing capacitor 414, and an oscillation signal (in a low frequency band, for example, from 1250 MHz to 1300 MHz), which corresponds to the parallel resonance frequency, is generated to the amplifier stage 42 and supplied to the oscillation signal output terminal 44 from the output terminal thereof.
The voltage controlled oscillator used in the improved-type hand-held phone, which can selectively output local oscillation signals in the two frequency bands, that is, in the high frequency band and in the low frequency band, by using the one voltage-controlled oscillator 31 and switching the LC resonator circuit 41 of the voltage controlled oscillator 31, employs the one voltage controlled oscillator 31 and the one loop filter 32, thereby employing the one phase locked loop. As a result, the voltage controlled oscillator can avoid an increase in size of the commonly usable hand-held phone as well as can prevent an increase in the manufacturing cost thereof.
However, the voltage controlled oscillator used in the improved-type hand-held phone employs the one loop filter 32 with respect to the two oscillation frequency bands of the high and low frequency bands as well as selectively outputs local oscillation signals of the two oscillation frequency bands by switching the LC resonator circuit 41. Thus, when the rate of change of an oscillation frequency Kv per unit control voltage (which is hereinafter referred to as a rate of change Kv) in the range in which the control voltage is used is determined as to the two oscillation frequency bands, the rate of change Kv in the low frequency band is in the same level as that in the high frequency band. As a result, it is difficult to make a difference between the rate of change Kv in the high frequency band and the rate of change Kv in the low frequency band, which exhibits undesirable characteristics when two discrete oscillation frequencies are controlled in the phase locked loop (PLL).
These undesirable characteristics can be avoided by dividing the loop filter 32 for supplying the control voltage to the voltage controlled oscillator 31 into two loop filters, that is, into a first loop filter for supplying a control voltage for the high frequency band and a second loop filter for supplying a control voltage for the low frequency band and by setting the filter characteristics of the first and second loop filters so as to increase a difference between the corresponding rates of change Kv. However, the provision of the two loop filters makes it difficult to avoid the increase in size and manufacturing cost of the commonly usable hand-held phone.
Accordingly, it is an object of the present invention, which was made in view of the above technical background, to provide a voltage controlled oscillator capable of increasing a difference between rates of change Kv in high and low frequency bands when an oscillation signal of the high or low frequency band is selectively outputted by switching an LC resonator circuit even if one loop filter is used.
To achieve the above object, in a voltage controlled oscillator according to the present invention including an LC resonator circuit that is connected in a phase locked loop having a phase comparator and a loop filter and sets oscillation frequencies as well as switching oscillation frequency bands by supplying a band switching signal to the LC resonator circuit, the LC resonator circuit includes a means having at least two inductor elements or capacitor elements and a capacitor element in which a voltage variable capacitor element is connected in series to a composite impedance element, a part of the at least two inductor elements or capacitor elements being switched to connection and non-connection in response to the band switching signal, the capacitance value of the voltage variable capacitor element being adjusted by the control voltage from the loop filter, and the composite impedance element exhibiting a capacitance value in oscillation frequency ranges.
According to the above-mentioned means, since the capacitor element, in which the voltage variable capacitor element is connected in series to the composite impedance element that exhibits the capacitance value in the oscillation frequency ranges, is used as the capacitor element of the LC resonator for setting oscillation frequencies, a rate of change Kv in a high oscillation frequency band is made small as a whole as well as a rate of change Kv in a low oscillation frequency band is made small as a whole. As a result, it is possible to provide a large difference between a changing state of the rate of change Kv in the high oscillation frequency band and a changing state of the rate of change Kv in the low oscillation frequency band, whereby the oscillation frequencies of the two discrete oscillation frequency bands can be excellently controlled by the control voltage even if the only one loop filter is used.
In this case, it is preferable that the composite impedance element used in the voltage controlled oscillator of the present invention be a series circuit of a direct current preventing capacitor and an inductor element having a fine inductance value.
With this arrangement, the composite impedance element for increasing the difference between the rates of change Kv in the high and low frequency bands can be realized by a very simple arrangement.